Is Klinefelter syndrome a trisomy?

Yes. Klinefelter's syndrome is a trisomy.

Klinefelter's syndrome

• It is a recessive disorder affecting males.
• Klinefelter's syndrome is a trisomy because usually in males there will be one X chromosome and Y chromosome. But the person affected with this syndrome will have an extra X chromosome making the chromosome pattern as XXY.
• Risk includes breast cancer, infertility for a male having this symptom.